By convention, glutamyltransferase (GT) activity has generally been equated with and expressed as glutamine synthetase (GS) activity. However, since recent studies indicate that GT and GS activities are, in some instances, separable and therefore that these reactions may be catalyzed by different enzymes, it now seems prudent to express GT activity only as a measure of the action of glutamyltransferase. High GT activities are preferentially associated with each component of an ascending visual pathway in the chick embryo including the neural retina, the optic tectum, the nucleus rotundus of the diencephalon and the ectostriatum of the cerebral hemisphere. To determine a developmental role for the enzyme in this embryonic neural tract, GT activities were localized first in cell populations and then in subcellular fractions of whole embryonic cerebral hemispheres. Briefly the results suggest that GT development is an event relating generally to neurons rather than supportive cells and that GT activities increase most significantly in the synaptosomal fraction. It is now important to demonstrate the precise distribution of the enzyme in nuclei of the visual pathway and appropriate "non-visual" areas of embryonic brain. It is assumed, at this time, that these studies will provide evidence of a unique association of the enzyme with synaptic events in the visual tract.